"RISC OS Open are very pleased to announce the official release of RISC OS for the Raspberry Pi, 'RISC OS Pi'. This is a watershed moment for RISC OS and represents the culmination of many months of hard work from a whole community of developers, testers and other contributors. It also means the Raspberry Pi can now boast support for the quick, compact, original ARM-based operating system." This is absolutely fantastic news. I'm going to try this on my Pi later this week.

That would be the PandaBoard ES, which has a dual core Cortex-A9 at 1.2 GHz. (There is also a 1.0 GHz PandaBoard that the ES effectively replaced.) However, RISC OS doesn't support multiprocessor systems, so one core stays idle. I suspect the PandaBoard also has the fastest floating point, as the TI OMAP4's version of the Cortex-A9 core has the VFPv3 unit included.

Also, the BeagleBoard xM (1 GHz Cortex-A8) and BeagleBoard (600 MHz Cortex-A8) are both faster than the Raspberry Pi's older 700 MHz ARM1176 for integer work. However, the Raspberry Pi is faster for floating point, as it seems that RISC OS has standardized on using VFP for floating point, instead of NEON (ARM backpedaled on the whole "deprecating VFP" thing that they tried with Cortex-A8, now it's just that the vector VFP instructions are deprecated, and NEON is used for those), and the Cortex-A8 uses a badly crippled VFP unit.

And if RISC OS supported it, which it doesn't (and usually a stable port isn't available until the platform's had about 6-12 months of RISC OS development, based on the past).

Fastest single ARM core you can readily buy is probably the Cortex-A15 as used in Samsung's Exynos 5 Dual, running at 1.7 GHz. (The second core sits there and does nothing under RISC OS.)

Once the X-Gene comes out, that thing will be an absolute beast. As far as I can tell, ARM basically outsourced the development of the first ARMv8 chips to AppliedMicro, and the X-Gene looks like it'll whip up on the Cortex-A15. And with eight 2.5 GHz cores. (About a third as fast as an equivalent Sandy Bridge core, it seems, but that's still REALLY FREAKING FAST as far as ARM stuff goes.) But, it's a server chip.